In its natural state, Castform is small, gray and resembles a cloud. When it is extremely sunny, Castform's lower half becomes pure white while the head turns orange with smaller orange balls surrounding it. This gives Castform's head a shape that is similar to the sun. During rain, Castform's lower half becomes a darker shade of gray. The head is blue, resembling a drop of rain. In hail or snowy weather, Castform's head turns purplish-blue. The rest of of the body changes into a mint green cumulonimbus cloud that also appears to resemble a decorative ice sculpture. Castform does not have a form for sandstorms or fog.

In all forms, Castform has markings around its eyes similar to a pair of sunglasses.

In the manga

A Castform, nicknamed Popo, belongs to Ruby in Pokémon Adventures. Previously, she belonged to Mr. Stone, who was injured in an attack by Team Aqua. Popo was then given to Gabby and Ty to be cared for while he was being healed. She later decided to travel with Ruby when he began traveling with the reporters because she was lonely. Until he was reunited with Ruru, Popo was Ruby's Smart contest Pokémon.

Despite several Pokédex entries stating that its molecules are just like water, it is not a Water-type Pokémon, though it does become one in rain. It may also be a reference to the fact that it resembles a living cloud.

Despite being noted for learning many moves that take advantage of or influence the weather, Castform cannot learn Mist, Haze, Heat Wave or Hurricane.

Origin

It appears to be intended to resemble and act like a living cloud. Its ability to change shape and type depending on the weather condition makes similar to a weather balloon. It's also similar both in concept and design to a teru teru bozu, a Japanese charm doll used to wish for good weather.

Name origin

Castform's name seems to be a combination of forecast, predict the weather, and transform, referring to its ability to change type depending on the weather. The German and French names refer to this as well.

Powalen may be a combination of 天気予報 tenkiyohō, weather forecast, and 変わる kawaru, to change or transform.